Method and system for matching air flow in an exhaust gas recirculation system

ABSTRACT

Various methods and systems are provided for an exhaust gas recirculation system. In one embodiment, an engine method comprises routing exhaust gas from a first cylinder group of an engine to an exhaust gas recirculation passage coupled to both an intake passage and an exhaust passage of the engine, the first cylinder group having a first amount of positive intake and exhaust valve overlap, and routing exhaust gas from a second cylinder group of the engine only to the exhaust passage of the engine, the second cylinder group having a second, smaller amount of positive intake and exhaust valve overlap.

FIELD

Embodiments of the subject matter disclosed herein relate to an engine,engine components, and an engine system, for example.

BACKGROUND

Engine valve event timing, such as the opening and closing of theexhaust valves, may be optimized to provide desired cylinder air flow,residual exhaust fraction, and other parameters. The timing of the valveevents may be similar for all cylinders, even if the cylindersexperience differential exhaust backpressure or additional factors thatmay impact cylinder air flow. Thus, while the valve timing may beoptimal for some cylinders, other cylinders may suffer from inefficientair flow, contributing to degraded fuel economy and/or emissions.

BRIEF DESCRIPTION

In one embodiment, an engine method comprises routing exhaust gas from afirst cylinder group of an engine to an exhaust gas recirculationpassage coupled to both an intake passage and an exhaust passage of theengine, the first cylinder group having a first amount of positiveintake and exhaust valve overlap, and routing exhaust gas from a secondcylinder group of the engine only to the exhaust passage of the engine,the second cylinder group having a second, smaller amount of positiveintake and exhaust valve overlap.

It should be understood that the brief description above is provided tointroduce in simplified form a selection of concepts that are furtherdescribed in the detailed description. It is not meant to identify keyor essential features of the claimed subject matter, the scope of whichis defined uniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood from reading thefollowing description of non-limiting embodiments, with reference to theattached drawings, wherein below:

FIG. 1 shows a schematic diagram of an example embodiment of an enginewith a plurality of donor cylinders and a plurality of non-donorcylinders according to an embodiment of the invention.

FIG. 2 shows a schematic diagram of one cylinder of an engine accordingto an embodiment of the invention.

FIG. 3 shows a schematic diagram of example cam lobe profiles accordingto an embodiment of the invention.

FIG. 4 is a flow chart illustrating a method for matching air flowbetween a plurality of donor cylinders and a plurality of non-donorcylinders according to an embodiment of the invention.

FIGS. 5A and 5B are diagrams depicting example intake and exhaust valvetimings according to an embodiment of the invention.

DETAILED DESCRIPTION

The following description relates to various embodiments of an exhaustgas recirculation system in an engine. In particular, in one embodiment,the exhaust gas recirculation system includes a first cylinder groupconfigured to route exhaust to an intake of the engine and/or toatmosphere. The first cylinder group includes a first camshaft to openand close the exhaust valves of the first cylinder group such that theexhaust valves are open for a first duration. A second cylinder group isconfigured to route exhaust only to atmosphere. The second cylindergroup includes a second camshaft configured to open the exhaust valvesof the second cylinder group for a second duration, shorter than thefirst duration, by closing the exhaust valves of the second cylindergroup earlier than the closing of the exhaust valves of the firstcylinder group. In one example, the duration may refer to a number ofcrankshaft angular degrees of rotation for which the exhaust valve isopen.

By opening the exhaust valves of the first cylinder group for adifferent duration than the second cylinder group, air flow between thebetween the cylinder groups may be better balanced and maintained withina tighter tolerance of one another, despite differences in cylinderconditions, such as due to difference in exhaust pressures. For example,the first cylinder group, because it may route exhaust back to theintake rather than atmosphere, may experience an increased amount ofexhaust backpressure and other factors that impact air flow into thecylinders, compared to the second cylinder group which only routesexhaust to the atmosphere. By differentially controlling the exhaustvalve events between the two cylinder groups, equal air flow to bothcylinder groups may be maintained.

While the above example illustrates different opening durations of theexhaust valves between the cylinder groups, or cylinder banks, otherdifferences in valve timing, lift, etc. may be used in addition, or inplace of, differences in the opening durations. For example, exhaustvalve opening timings, exhaust valve closing timings, exhaust valve liftamounts, and other parameters may be different between the cylindergroups.

FIG. 1 shows a block diagram of an exemplary embodiment of an enginesystem 100 with an engine 104, such as an internal combustion engine.The engine 104 receives intake air for combustion from an intake, suchas an intake manifold 115. The intake may be any suitable conduit orconduits through which gases flow to enter the engine. For example, theintake may include the intake manifold 115, an intake passage 114, andthe like. The intake passage 114 receives ambient air from an air filter(not shown) that filters air from outside of a vehicle in which theengine 104 may be positioned. Exhaust gas resulting from combustion inthe engine 104 is supplied to an exhaust, such as exhaust passage 116.The exhaust may be any suitable conduit through which gases flow fromthe engine. For example, the exhaust may include an exhaust manifold117, the exhaust passage 116, and the like. Exhaust gas flows throughthe exhaust passage 116.

In the example embodiment depicted in FIG. 1, the engine 104 is a V-12engine having twelve cylinders. In other examples, the engine may be aV-6, V-8, V-10, V-16, I-4, I-6, I-8, opposed 4, or another engine type.As depicted, the engine 104 includes a subset of non-donor cylinders105, which includes six cylinders that supply exhaust gas exclusively toa non-donor cylinder exhaust manifold 117, and a subset of donorcylinders 107, which includes six cylinders that supply exhaust gasexclusively to a donor cylinder exhaust manifold 119. In otherembodiments, the engine may include at least one donor cylinder and atleast one non-donor cylinder. For example, the engine may have fourdonor cylinders and eight non-donor cylinders, or three donor cylindersand nine non-donor cylinders. It should be understood, the engine mayhave any desired numbers of donor cylinders and non-donor cylinders,with the number of donor cylinders typically lower than the number ofnon-donor cylinders.

As depicted in FIG. 1, the non-donor cylinders 105 are coupled to theexhaust passage 116 to route exhaust gas from the engine to atmosphere(after it passes through an exhaust gas treatment system 130 and firstand second turbochargers 120 and 124). The donor cylinders 107, whichprovide engine exhaust gas recirculation (EGR), are coupled exclusivelyto an EGR passage 162 of an EGR system 160 which routes exhaust gas fromthe donor cylinders 107 to the intake passage 114 of the engine 104, andnot to atmosphere. By introducing cooled exhaust gas to the engine 104,the amount of available oxygen for combustion is decreased, therebyreducing combustion flame temperatures and reducing the formation ofnitrogen oxides (e.g., NO_(x)).

Thus, the engine includes a first, donor cylinder group configured toroute exhaust to the intake and/or atmosphere, and a second, non-donorcylinder group configured to route exhaust only to atmosphere. Thenon-donor cylinder exhaust manifold 117 and donor cylinder exhaustmanifold 119 are maintained separately from each other. Other than thecross-over passage controlled by valve 164, the manifolds do not includecommon passageways enabling communication between the non-donor cylindermanifold and the donor cylinder manifold. However, both the first, donorcylinder group and second, non-donor cylinder group receive the sameintake air via the intake manifold 115, and are subject to equal intakemanifold pressure.

In the example embodiment shown in FIG. 1, when a second valve 170 isopen, exhaust gas flowing from the donor cylinders 107 to the intakepassage 114 passes through a heat exchanger such as an EGR cooler 166 toreduce a temperature of (e.g., cool) the exhaust gas before the exhaustgas returns to the intake passage. The EGR cooler 166 may be anair-to-liquid heat exchanger, for example. In such an example, one ormore charge air coolers 132 and 134 disposed in the intake passage 114(e.g., upstream of where the recirculated exhaust gas enters) may beadjusted to further increase cooling of the charge air such that amixture temperature of charge air and exhaust gas is maintained at adesired temperature. In other examples, the EGR system 160 may includean EGR cooler bypass. Alternatively, the EGR system may include an EGRcooler control element. The EGR cooler control element may be actuatedsuch that the flow of exhaust gas through the EGR cooler is reduced;however, in such a configuration, exhaust gas that does not flow throughthe EGR cooler is directed to the exhaust passage 116 rather than theintake passage 114.

Further, the EGR system 160 includes a first valve 164 disposed betweenthe exhaust passage 116 and the EGR passage 162. The second valve 170may be an on/off valve controlled by the control unit 180 (for turningthe flow of EGR on or off), or it may control a variable amount of EGR,for example. In some examples, the first valve 164 may be actuated suchthat an EGR amount is reduced (exhaust gas flows from the EGR passage162 to the exhaust passage 116). In other examples, the first valve 164may be actuated such that the EGR amount is increased (e.g., exhaust gasflows from the exhaust passage 116 to the EGR passage 162). In someembodiments, the EGR system 160 may include a plurality of EGR valves orother flow control elements to control the amount of EGR.

In such a configuration, the first valve 164 is operable to routeexhaust from the donor cylinders to the exhaust passage 116 of theengine 104 and the second valve 170 is operable to route exhaust fromthe donor cylinders to the intake passage 114 of the engine 104. Assuch, the first valve 164 may be referred to as an exhaust valve, whilethe second valve 170 may be referred to as an EGR valve. In the exampleembodiment shown in FIG. 1, the first valve 164 and the second valve 170may be engine oil, or hydraulically, actuated valves, for example, witha shuttle valve (not shown) to modulate the engine oil. In someexamples, the valves may be actuated such that one of the first andsecond valves 164 and 170 is normally open and the other is normallyclosed. In other examples, the first and second valves 164 and 170 maybe pneumatic valves, electric valves, or another suitable valve.

As shown in FIG. 1, the engine system 100 further includes an EGR mixer172 which mixes the recirculated exhaust gas with charge air such thatthe exhaust gas may be evenly distributed within the charge air andexhaust gas mixture. In the example embodiment depicted in FIG. 1, theEGR system 160 is a high-pressure EGR system which routes exhaust gasfrom a location upstream of turbochargers 120 and 124 in the exhaustpassage 116 to a location downstream of turbochargers 120 and 124 in theintake passage 114. In other embodiments, the engine system 100 mayadditionally or alternatively include a low-pressure EGR system whichroutes exhaust gas from downstream of the turbochargers 120 and 124 inthe exhaust passage 116 to a location upstream of the turbochargers 120and 124 in the intake passage 114.

Engine system 100 may include one or more camshafts to control intakeand/or exhaust valve events. As shown in FIG. 1, a first camshaft 140controls exhaust valve events for the donor cylinder group and a secondcamshaft 142 controls exhaust valve events for the non-donor cylindergroup. The first camshaft 140 may include cam lobes having a firstprofile to close the exhaust valves of the donor cylinder group at afirst timing. The second camshaft 142 may include cam lobes having asecond profile to close the exhaust valves of the non-donor cylindergroup at a second timing. The second timing may be earlier than thefirst timing. Example cam lobe profiles for cams lobes of the first andsecond camshafts are depicted in FIG. 3.

In some embodiments, the first camshaft 140 may be configured to openthe exhaust valves of the donor cylinder group for a first duration,while the second camshaft 142 may be configured to open the exhaustvalves of the non-donor cylinder group for a second duration, differentthan the first duration. Engine system 100 may include additionalcamshafts to control intake valve events, or first and second camshafts140, 142 may control both intake and exhaust valve events. Additionalinformation regarding controlling exhaust valve events will be presentedbelow with respect to FIGS. 2-5.

As depicted in FIG. 1, the engine system 100 further includes atwo-stage turbocharger with the first turbocharger 120 and the secondturbocharger 124 arranged in series, each of the turbochargers 120 and124 arranged between the intake passage 114 and the exhaust passage 116.The two-stage turbocharger increases air charge of ambient air drawninto the intake passage 114 in order to provide greater charge densityduring combustion to increase power output and/or engine-operatingefficiency. The first turbocharger 120 operates at a relatively lowerpressure, and includes a first turbine 121 which drives a firstcompressor 122. The first turbine 121 and the first compressor 122 aremechanically coupled via a first shaft 123. The second turbocharger 124operates at a relatively higher pressure, and includes a second turbine125 which drives a second compressor 126. The second turbine and thesecond compressor are mechanically coupled via a second shaft 127. Inthe example embodiment shown in FIG. 1, the second turbocharger 124 isprovided with a wastegate 128 which allows exhaust gas to bypass thesecond turbocharger 124. The wastegate 128 may be opened, for example,to divert the exhaust gas flow away from the second turbine 125. In thismanner, the rotating speed of the compressors 126, and thus the boostprovided by the turbochargers 120, 124 to the engine 104 may beregulated during steady state conditions. In other embodiments, each ofthe turbochargers 120 and 124 may be provided with a wastegate, or onlythe second turbocharger 124 may be provided with a wastegate.

The engine system 100 further includes an exhaust treatment system 130coupled in the exhaust passage in order to reduce regulated emissions.As depicted in FIG. 1, the exhaust gas treatment system 130 is disposeddownstream of the turbine 121 of the first (low pressure) turbocharger120. In other embodiments, an exhaust gas treatment system may beadditionally or alternatively disposed upstream of the firstturbocharger 120. The exhaust gas treatment system 130 may include oneor more components. For example, the exhaust gas treatment system 130may include one or more of a diesel particulate filter (DPF), a dieseloxidation catalyst (DOC), a selective catalytic reduction (SCR)catalyst, a three-way catalyst, a NO_(x) trap, and/or various otheremission control devices or combinations thereof.

The engine system 100 further includes the control unit 180, which isprovided and configured to control various components related to theengine system 100. In one example, the control unit 180 includes acomputer control system. The control unit 180 further includesnon-transitory, computer readable storage media including code forenabling on-board monitoring and control of engine operation. Thecontrol unit 180, while overseeing control and management of the enginesystem 100, may be configured to receive signals from a variety ofengine sensors, as further elaborated herein, in order to determineoperating parameters and operating conditions, and correspondinglyadjust various engine actuators to control operation of the enginesystem 100. For example, the control unit 180 may receive signals fromvarious engine sensors including, but not limited to, engine speed,engine load, boost pressure, ambient pressure, exhaust temperature,exhaust pressure, etc. Correspondingly, the control unit 180 may controlthe engine system 100 by sending commands to various components such astraction motors, alternator, cylinder valves, throttle, heat exchangers,wastegates or other valves or flow control elements, etc.

FIG. 2 depicts an example embodiment of a combustion chamber, orcylinder 200, of a multi-cylinder internal combustion engine, such asthe engine 104 described above with reference to FIG. 1. The cylinder(i.e., combustion chamber) 200 may include combustion chamber walls 204with a piston 206 positioned therein. The piston 206 may be coupled to acrankshaft 208 so that reciprocating motion of the piston is translatedinto rotational motion of the crankshaft. In some embodiments, theengine may be a four-stroke engine in which each of the cylinders firesin a firing order during two revolutions of the crankshaft 208. In otherembodiments, the engine may be a two-stroke engine in which each of thecylinders fires in a firing order during one revolution of thecrankshaft 208. Cylinder 200 may include a cooling sleeve 228 which mayinclude a temperature sensor 230 configured to output an engine coolanttemperature (ECT) signal to control unit 180.

The cylinder 200 receives intake air for combustion from an intakepassage 210. The intake passage 210 receives ambient air from an airfilter (not shown) that filters air from outside of a vehicle in whichthe engine is positioned. The intake passage 210 may communicate withother cylinders of the engine in addition to the cylinder 200, forexample.

Exhaust gas resulting from combustion in the engine is supplied to anexhaust passage 212. Exhaust gas flows through the exhaust passage 212,to a turbocharger (not shown in FIG. 2) and to atmosphere. The exhaustpassage 212 may further receive exhaust gases from other cylinders ofthe engine in addition to the cylinder 200, for example.

In some embodiments, as described with reference to FIG. 1, the vehiclesystem may include more than one exhaust passage. For example, a donorgroup of cylinders may be coupled to a first exhaust manifold and anon-donor group of cylinders may be coupled to a second exhaust manifoldthat is separated from the first exhaust manifold. In this way, one ofthe groups of cylinders may be comprised exclusively of donor cylinderswhich recirculate exhaust gas to the intake passage 210 under selectedoperating conditions.

Continuing with FIG. 2, each cylinder of the engine may include one ormore intake valves and one or more exhaust valves. For example, thecylinder 200 is shown including at least one intake poppet valve 214 andat least one exhaust poppet valve 216 located in an upper region ofcylinder 200. In some embodiments, each cylinder of the engine,including cylinder 200, may include at least two intake poppet valvesand at least two exhaust poppet valves located at the cylinder head.

In some embodiments, the intake valve 214 may be controlled by thecontrol unit 180 via an actuator 218. Similarly, the exhaust valve 216may be controlled by the control unit 180 via an actuator 220. Duringsome conditions, the control unit 180 may vary the signals provided tothe actuators 218 and 220 to control the opening and closing of therespective intake and exhaust valves relative to crankshaft timing. Theposition of the intake valve 214 and the exhaust valve 216 may bedetermined by respective valve position sensors 222 and 224,respectively. The valve actuators may be of the electric valve actuationtype or cam actuation type, or a combination thereof, for example. Inother embodiments, the intake valve 214 and exhaust valve 216 may becontrolled via a fixed camshaft timing system relative to crankshafttiming rather than via the control unit 180.

The intake and exhaust valve timing may be controlled concurrently orany of a possibility of variable intake cam timing, variable exhaust camtiming, dual independent variable cam timing or fixed cam timing may beused. In other embodiments, the intake and exhaust valves may becontrolled by a common valve actuator or actuation system, or a variablevalve timing actuator or actuation system. Further, the intake andexhaust valves may by controlled to have variable lift by the controllerbased on operating conditions.

In some embodiments, each cylinder of the engine may be configured withone or more fuel injectors for providing fuel thereto. As a non-limitingexample, FIG. 2 shows the cylinder 200 is including a fuel injector 226.The fuel injector 226 is shown coupled directly to the cylinder 200 forinjecting fuel directly therein. In this manner, fuel injector 226provides what is known as direct injection of a fuel into combustioncylinder 200. The fuel may be delivered to the fuel injector 226 from ahigh-pressure fuel system including a fuel tank, fuel pumps, and a fuelrail (not shown). In one example, the fuel is diesel fuel that iscombusted in the engine through compression ignition. In othernon-limiting embodiments, the second fuel may be gasoline, kerosene,biodiesel, or other petroleum distillates of similar density throughcompression ignition (and/or spark ignition).

Turning to FIG. 3, a diagram 300 depicting example cam lobes is shown. Afirst cam lobe 302 having a first cam profile is depicted with a dashedline and superimposed over a second cam lobe 304 having a second camprofile, where the cams in this end view are positioned at the samecrank angle timing. The first cam lobe 302 may be included on firstcamshaft 140 in order to control the exhaust valve events of the first,donor cylinder group. The second cam lobe 304 may be included on secondcamshaft 142 in order to control the exhaust valve events of the second,non-donor cylinder group. As shown in FIG. 3, the first cam lobe has awider tip end 306 as compared to the tip end 308 of the second cam lobe304. This results in the exhaust valve for which cam lobe 302 controlsto be open for a longer duration and close later than the exhaust valvefor which cam lobe 304 controls. Both cam lobes 302 and 304 areconfigured to open exhaust valves at the same time. However, in someembodiments, cam lobe 302 may have an end tip with a width substantiallyequal to the width of the tip end of cam lobe 304, and may be rotated atan angle with respect to cam lobe 304 in order to open and close itsexhaust valve later than cam lobe 304 relative to crank angle and thuspiston position relative to top-dead-center and/or bottom-dead-center.

The engine system as illustrated in FIGS. 1 and 2 may be configured forimproved air flow into both the donor and non-donor cylinders. Theexhaust valve timing for the donor cylinders may be different than theexhaust valve timing of the non-donor cylinders in order to provideimproved air flow to the donor cylinders. The exhaust valves of thedonor cylinders may be closed later than the exhaust valves of thenon-donor cylinders and hence provide an increased amount of positivevalve overlap between the intake and exhaust valves of the donorcylinder group as compared to the non-donor cylinders. The delayedexhaust valve closing of the donor cylinders may be provided by a fixedcamshaft that is structured different than the camshaft controlling thevalve events of the non-donor cylinders, such as shown in FIG. 3.However, in other embodiments, the delayed exhaust valve closing may beprovided by a variable camshaft timing system that is controlled by thecontrol unit 180 to adjust the timing of the exhaust valve closing basedon operating conditions.

Thus, in some embodiments an engine system includes a first cylindergroup coupled exclusively to an exhaust gas recirculation passageconfigured to route exhaust to an intake passage and an exhaust passageof the engine, a second cylinder group coupled exclusively to theexhaust passage of the engine, a first camshaft having cam lobes with afirst profile to close exhaust valves of the first cylinder group at afirst timing, and a second camshaft having cam lobes with a secondprofile to close exhaust valves of the second group at a second timing,the second timing earlier than the first timing.

In another embodiment, a system comprises means for routing exhaust gasfrom a first cylinder group of an engine to an exhaust gas recirculationpassage coupled to both an intake passage and an exhaust passage of theengine, the first cylinder group having a first amount of positiveintake and exhaust valve overlap, and means for routing exhaust gas froma second cylinder group of the engine only to the exhaust passage of theengine, the second cylinder group having a second, smaller amount ofpositive intake and exhaust valve overlap.

In one example, the means for routing exhaust gas from a first cylindergroup to an exhaust recirculation passage coupled to both an intakepassage and exhaust passage comprises a first exhaust manifold, such asexhaust manifold 119, coupled to an EGR passage, such as passage 162.Thus, when the exhaust valves of the first cylinder group open, theexhaust is expelled to exhaust manifold 119 and then to EGR passage 162.Based on the positions of first valve 164 and second valve 170, theexhaust may be routed to the intake and/or to atmosphere. Otherconfigurations are also possible for the means for routing exhaust fromthe first cylinder group to an exhaust recirculation passage coupled toboth an intake passage and an exhaust passage. For example, the meansmay comprise multiple exhaust manifolds, each coupled to a common EGRpassage or to separate EGR passages. In another example, each cylindermay include separate cylinder exhaust passages rather than a manifold,and each cylinder exhaust passage may be coupled to the EGR passage.

In a further example, the means for routing exhaust gas from the secondcylinder group of the engine only to the exhaust passage of the enginecomprises a second exhaust manifold, such as manifold 117, coupled to anexhaust passage, such as passage 116. When the exhaust valves of thesecond cylinder group open, the exhaust is expelled to the secondexhaust manifold, through the exhaust passage to the atmosphere. Similarto the means described above, other configurations are possible, such asmultiple exhaust manifolds, or separate exhaust passages rather than amanifold.

Turning to FIG. 4, a flow chart illustrating a method 400 for matchingair flow rates between cylinder groups is depicted. Method 400 may becarried out by control unit 180 according to instructions storedthereon. Method 400 may provide for differential exhaust valve timingbetween a first cylinder group, configured to route exhaust back to theintake, and a second cylinder group, configured to route exhaust only toatmosphere. The differential exhaust valve timing may be activated inresponse to designated operating conditions. The designated operatingconditions may be indicative of EGR being routed from the first cylindergroup to the intake of the engine. By activating differential exhaustvalve timing in response to EGR, air flow through both the first andsecond cylinder groups may be optimized to provide enhanced performanceand allow for equal fuel distribution among the cylinders.

Method 400 comprises, at 402, determining engine operating parameters.The engine operating parameters may include engine speed and load,engine temperature, EGR valve position (such as the position of valve170), the amount of exhaust being routed to the intake, and otherparameters. Based on the operating parameters, at 403, one or more ofthe exhaust valve (e.g., valve 164) and EGR valve (e.g., valve 170) maybe adjusted to deliver a desired amount of EGR to the cylinders. Forexample, during low engine speed and load or cold engine temperatureconditions, no EGR may be routed to the cylinders, and as such the EGRvalve may be closed and the exhaust valve may be opened. During higherspeed and load conditions, the EGR valve may be opened to deliverexhaust from the first, donor cylinder group to the engine. Based on thedesired amount of EGR, which may be a function of engine speed and load,the exhaust valve may be opened or closed. Further, if the EGR valve andexhaust valve include more than two positions, the EGR and/or exhaustvalve may be set to a partially open position to deliver a desiredamount of EGR to the engine. The exhaust from the first cylinder groupthat is not routed to the engine is routed to the exhaust passage andeventually to atmosphere.

At 404 it is determined if designated conditions have been met. In someembodiments, the designated conditions may include the EGR valve beingin an open position. In other embodiments, the designated conditions mayinclude an amount of exhaust recirculated to the intake exceeding athreshold, such as an EGR rate of 5%. In still further embodiments, thedesignated conditions may include engine speed and load exceedingrespective thresholds. For example, the designated conditions mayinclude mid to high engine speed and load.

If the designated conditions have not been met, method 400 proceeds to406 to open and close the exhaust valves of the first and secondcylinder groups at equal timings. Because conditions indicating EGR flowto the intake are not met, both cylinder groups may be experiencingsimilar cylinder air flow conditions. Hence, the intake and exhaustvalves may be opened and closed equally between the first and secondcylinder groups. If the designated conditions have been met, method 400proceeds to 408 to open the exhaust valves of the first and secondcylinder groups at equal timings.

At 410, method 400 includes closing the exhaust valves of the first andsecond cylinder groups at different timings. As explained previously, byopening the exhaust valves of the first cylinder group for a differentduration than the second cylinder group, air flow through the firstcylinder group may be optimized to compensate for the additional exhaustbackpressure and other factors caused by the routing of exhaust from thefirst cylinder group to the intake of the engine.

Closing the exhaust valves of the first and second cylinder groups atdifferent timings may include closing the exhaust valves of the firstcylinder group at a first timing at 412. At 414, closing the exhaustvalves at different timings may also include closing the exhaust valvesof the second cylinder group at a second, earlier timing. In this way,the exhaust valves of the first cylinder group, which donates exhaustfor EGR, may be closed later the exhaust valves of the second cylindergroup, which does not donate exhaust for EGR. The exhaust valves of thefirst cylinder group may be opened for a first duration and the exhaustvalves of the second cylinder group may be opened for no more than asecond, shorter duration between successive valve closings.

At 416, closing the exhaust valves at different timings may includeadjusting a variable camshaft timing (VCT) system to delay the closingof the valves of the first cylinder group. Thus, by utilizing a VCTsystem, the valve timing of the first, donor cylinder group may beadjusted based on operating conditions in order to maintain optimal airflow through the cylinder group. However, in some embodiments, a fixedcamshaft timing system may be used. In such circumstances, the valveevents of the first cylinder group may not be adjusted based onoperating conditions. Instead, regardless of whether EGR is routed tothe intake from the first cylinder group or whether the exhaust from thefirst cylinder group is routed to the atmosphere, the exhaust valves ofthe first cylinder group may be closed later than the second cylindergroup, during substantially all engine operating conditions. As usedherein, substantially all engine operating conditions comprises alloperating conditions where valve events are occurring.

At 418, method 400 optionally includes opening and closing the intakevalves of the first and second cylinder groups at equal timings. Thus,only the exhaust valve events may be different between the first andsecond cylinder groups. In other embodiments, however, the intake valvetiming of the first cylinder group may be adjusted similarly to theexhaust valve timing, such that the intake valves of the first cylindergroup are closed later than the intake valves of the second cylindergroup.

At 420, method 400 optionally includes distributing fuel equally amongall cylinders. Because the air flow between the first and secondcylinder groups is matched as a result of the delayed exhaust valveclosing, equal fuel amounts may be injected to each cylinder in order tomaintain the cylinders at relatively equal air-fuel ratios. However, inother embodiments, the fuel may be distributed differentially among thecylinders. For example, the first cylinder group may receive anincreased amount of fuel compared to the second cylinder group.Alternatively or additionally, fuel injection parameters, including theamount of fuel injected to each cylinder as well as the injectiontiming, may be maintained upon the adjustment of the exhaust valveclosing timing.

At 422, method 400 includes adjusting the exhaust valve and/or EGR valveif indicated. After delaying the closing time of the exhaust valves ofthe first cylinder group, the exhaust back pressure from the firstcylinder group may be different than the pressure from the secondcylinder group. For example, adjusting the valve closing time may resultin decreased exhaust pressure from the first cylinder group relative tothe second cylinder group. The exhaust and EGR valves may optionally beadjusted based on this pressure differential. For example, the exhaustvalve may be closed as the exhaust pressure from the first cylindergroup decreases, in order to increase the pressure of the exhausttraveling to the intake.

Thus, the method of FIG. 4 provides for an engine method comprising,during designated conditions, matching air flow rates through a firstcylinder group to air flow rates through a second cylinder group,selectively routing exhaust from the first cylinder group to an exhaustpassage and to an intake passage, and routing exhaust from the secondcylinder group only to the exhaust passage. The air flow rates may bematched by closing exhaust valves of the first cylinder group at a firsttiming and closing exhaust valves of the second cylinder group at anearlier, second timing.

In another embodiment, a method includes opening exhaust valves of afirst cylinder group for a first duration, opening exhaust valves of asecond cylinder group for no more than a second, shorter durationbetween successive valve closings, selectively routing exhaust from athe first cylinder group to an exhaust passage via a first valve and toan intake passage via a second valve, and routing exhaust from thesecond cylinder group only to the exhaust passage.

In this way, the exhaust valves of the first cylinder group may beclosed at a timing later than the exhaust valve closing timing of thesecond cylinder group. FIGS. 5A and 5B are diagrams depicting examplevalve timing events for the first and second cylinder groups. FIG. 5Aillustrates valve timing events for the second cylinder group in diagram500 while FIG. 5B illustrates the valve timing events for the firstcylinder group in diagram 510. Both diagram 500 and diagram 510 includeengine position on the x axis and valve lift on the y axis. Referring todiagram 500 of FIG. 5A, curves 502 and 504 depict valve timings for anexhaust valve (dashed curve 502) and an intake valve (solid curve 504)of the second, non-EGR donating cylinder group during the same engineoperation and at the same scale, although the different cylinders may bestaggered with respect to engine position according to the firing orderof the engine. As illustrated, an exhaust valve may be opened just asthe piston bottoms out at the end of the power stroke. The exhaust valvemay then close as the piston completes the exhaust stroke, remainingopen at least until a subsequent intake stroke has commenced. In thesame way, an intake valve may be opened at or before the start of anintake stroke, and may remain open at least until a subsequentcompression stroke has commenced.

As a result of the timing differences between exhaust valve closing andintake valve opening, for a short duration, before the end of theexhaust stroke and after the commencement of the intake stroke, bothintake and exhaust valves may be open. This period, during which bothvalves may be open, is referred to as a positive intake to exhaust valveoverlap 506 (or simply, positive valve overlap), represented by ahatched region at the intersection of curves 502 and 504.

Referring to diagram 510 of FIG. 5B, curves 512 and 514 depict valvetimings for an exhaust valve (dashed curve 512) and an intake valve(solid curve 514) of the first, EGR donating cylinder group duringengine operation. As illustrated, an exhaust valve may be opened just asthe piston bottoms out at the end of the power stroke. The exhaust valvemay then close as the piston completes the exhaust stroke, remainingopen at least until a subsequent intake stroke has commenced. In thesame way, an intake valve may be opened at or before the start of anintake stroke, and may remain open at least until a subsequentcompression stroke has commenced. However, in contrast the secondcylinder group, the exhaust valve of the first cylinder group may beclosed slightly later than the exhaust valve of the second cylindergroup, resulting in a larger amount of positive valve overlap 516.

Furthermore, in some embodiments, the exhaust valve of the firstcylinder group (curve 512) may open at the same time as the exhaustvalve of the second cylinder group, as depicted in FIGS. 5A and 5B.However, in other embodiments the exhaust valve of the first cylindergroup may open later than the exhaust valve of the second cylindergroup. Additionally, the maximum valve lift attained by the exhaustvalve of the second cylinder group, shown by dashed line 508, may beequal to the maximum valve lift attained by the exhaust valve of firstcylinder group (dashed line 518). In other embodiments, the maximumvalve lift of the exhaust valve of the first cylinder group may bedifferent than the exhaust valve of the second cylinder group. Forexample, if the exhaust valve of the first cylinder group is open for alonger duration than the exhaust valve of the second cylinder group, itmay have a lower maximum valve lift.

While the above-described system and method match air flow rates betweenthe donor and non-donor cylinder groups by utilizing differentialexhaust valve timing, other mechanisms for matching air flow rates arepossible. For example, the donor and non-donor cylinder groups may havedifferent compression ratios. The compression ratio of a given cylindermay be a function of cylinder bore diameter, cylinder volume, and pistonstoke, and thus the donor cylinders may include different cylinder borediameter, volume, and/or piston stroke relative to the non-donorcylinders. Additionally or alternatively, the effective compressionratio of a cylinder may be reduced by delaying closing of the cylinder'sintake valve, and thus the donor cylinder group may be configured tohave a different intake valve closing timing relative to the non-donorcylinder group.

In some embodiments, the donor cylinder group may be configured to havea decreased compression ratio relative to the non-donor cylinder group.By decreasing the compression ratio of the donor cylinders, a smalleramount of charge air may be provided to the donor cylinders, thusmatching the air flow rates to the non-donor cylinders. In otherembodiments, the donor cylinder group may be configured to have anincreased compression ratio relative to the non-donor cylinder group. Byincreasing the compression ratio, the donor cylinders may be able tobetter accommodate the cylinder exhaust fraction that may result fromthe increased exhaust back pressure provided by the routing of theexhaust to the intake.

Thus, in order to match air flow rates between the donor and non-donorcylinder groups, the exhaust valves of the donor cylinder group may beclosed at a timing different than the exhaust valves of the non-donorcylinder, the donor cylinders may have a different amount of valve liftthan the non-donor cylinders, or the donor cylinders may have adifferent compression ratio than the non-donor cylinders. Further,combinations of the above are also possible. For example, the donorcylinders may have a delayed exhaust valve closing timing and delayedintake valve closing timing relative to the non-donor cylinders. Inanother example, the donor cylinders may have a delayed exhaust valveclosing timing, increased compression ratio, and a different amount ofvalve lift than the non-donor cylinders.

In one embodiment, an engine method includes routing exhaust gas from afirst cylinder group of an engine to an exhaust gas recirculationpassage coupled to both an intake passage and an exhaust passage of theengine, the first cylinder group having a first compression ratio. Themethod includes routing exhaust gas from a second cylinder group of theengine only to the exhaust passage of the engine, the second cylindergroup having a second compression ratio, different than the firstcompression ratio. In one example, the compression ratio of the firstcylinder group may be greater than the compression ratio of the secondcylinder group. In another example, the compression ratio of the firstcylinder group may be less than the compression ratio of the secondcylinder group.

As explained above, the terms “high pressure” and “low pressure” arerelative, meaning that “high” pressure is a pressure higher than a “low”pressure. Conversely, a “low” pressure is a pressure lower than a “high”pressure.

As used herein, an element or step recited in the singular and proceededwith the word “a” or “an” should be understood as not excluding pluralof said elements or steps, unless such exclusion is explicitly stated.Furthermore, references to “one embodiment” of the present invention arenot intended to be interpreted as excluding the existence of additionalembodiments that also incorporate the recited features. Moreover, unlessexplicitly stated to the contrary, embodiments “comprising,”“including,” or “having” an element or a plurality of elements having aparticular property may include additional such elements not having thatproperty. The terms “including” and “in which” are used as theplain-language equivalents of the respective terms “comprising” and“wherein.” Moreover, the terms “first,” “second,” and “third,” etc. areused merely as labels, and are not intended to impose numericalrequirements or a particular positional order on their objects.

This written description uses examples to disclose the invention,including the best mode, and also to enable a person of ordinary skillin the relevant art to practice the invention, including making andusing any devices or systems and performing any incorporated methods.The patentable scope of the invention is defined by the claims, and mayinclude other examples that occur to those of ordinary skill in the art.Such other examples are intended to be within the scope of the claims ifthey have structural elements that do not differ from the literallanguage of the claims, or if they include equivalent structuralelements with insubstantial differences from the literal languages ofthe claims.

The invention claimed is:
 1. An engine method, comprising: adjusting anexhaust gas recirculation (EGR) valve to selectively route exhaust gasfrom a first cylinder group of an engine to one or both of an intakepassage and an exhaust passage of the engine via an exhaust gasrecirculation passage, including during a first condition, adjusting theEGR valve to a first position to route all exhaust gas from the firstcylinder group to the exhaust passage and during a second condition,adjusting the EGR valve to a second position to route at least someexhaust gas from the first cylinder group to the intake passage, andresponsive to the EGR valve being in the second position, the firstcylinder group has a first amount of positive intake and exhaust valveoverlap and responsive to the EGR valve being in the first position, thefirst cylinder group has a second, smaller amount of positive intake andexhaust valve overlap; and routing exhaust gas from a second cylindergroup of the engine only to the exhaust passage of the engine, thesecond cylinder group having the second, smaller amount of positiveintake and exhaust valve overlap, the exhaust gas recirculation passageconfigured to provide exhaust gas to both the first cylinder group andthe second cylinder group.
 2. The method of claim 1, wherein during thesecond condition, the first cylinder group has a delayed exhaust valveclosing timing with respect to the second cylinder group.
 3. The methodof claim 1, wherein the first cylinder group and the second cylindergroup have equal intake valve timing, wherein the EGR valve is a firstEGR valve fluidically coupling the EGR passage to the intake passage,and further comprising a second EGR valve fluidically coupling the EGRpassage to the exhaust passage, and during the first condition, thefirst EGR valve being in the first position includes the first EGR valvebeing fully closed and the second EGR valve is fully open and during thesecond condition, the first EGR valve being in the second positionincludes the first EGR valve being at least partly open.
 4. The methodof claim 1, wherein the first cylinder group has a delayed intake valveclosing timing with respect to the second cylinder group at least duringthe second condition.
 5. The method of claim 1, further comprisingdistributing fuel equally among all cylinders of the first and secondcylinder groups during at least the second condition.
 6. The method ofclaim 1, further comprising distributing a different amount of fuel tothe first cylinder group than to the second cylinder group.
 7. An enginemethod, comprising: operating an engine by distributing fuel to a firstcylinder group and to a second cylinder group; responsive to an exhaustgas recirculation valve being in an open position where exhaust gas isrouted from the first cylinder group to an exhaust passage and to anintake passage, increasing an amount of positive intake and exhaustvalve overlap for the first cylinder group from a first amount to asecond amount while maintaining the first amount of positive intake andexhaust valve overlap for the second cylinder group to match air flowrates through the first cylinder group to air flow rates through thesecond cylinder group; and routing exhaust from the second cylindergroup only to the exhaust passage.
 8. The method of claim 7, whereinincreasing the amount of positive intake and exhaust valve overlap forthe first cylinder group from the first amount to the second amountwhile maintaining the first amount of positive intake and exhaust valveoverlap for the second cylinder group comprises closing exhaust valvesof the first cylinder group at a first timing and closing exhaust valvesof the second cylinder group at an earlier, second timing to provideequal air flow rates through the first cylinder group and through thesecond cylinder group.
 9. The method of claim 8, further comprisingopening and closing intake valves of the first cylinder group at timingsequal to opening and closing of intake valves of the second cylindergroup.
 10. The method of claim 8, further comprising, if the exhaust gasrecirculation valve is in the open position, adjusting a variablecamshaft timing system to close the exhaust valves of the first cylindergroup later than the exhaust valves of the second cylinder group; and ifthe exhaust gas recirculation valve is in a closed position whereexhaust gas from the first cylinder group is routed only to the exhaustpassage, adjusting the variable camshaft timing system to close theexhaust valves of the first and second cylinder groups at equal timings.11. The method of claim 7, wherein the increasing the amount of positiveintake and exhaust valve overlap for the first cylinder group from thefirst amount to the second amount while maintaining the first amount ofpositive intake and exhaust valve overlap for the second cylinder groupis further responsive to one or more of an amount of exhaust routed fromthe first cylinder group to the intake passage exceeding a threshold orengine speed and load above respective thresholds.
 12. The method ofclaim 7, wherein the increasing the amount of positive intake andexhaust valve overlap for the first cylinder group from the first amountto the second amount while maintaining the first amount of positiveintake and exhaust valve overlap for the second cylinder group isfurther responsive to an exhaust backpressure of the first cylindergroup exceeding an exhaust backpressure of the second cylinder group.13. The method of claim 7, wherein distributing fuel to the firstcylinder group and the second cylinder group comprises distributing fuelequally among all cylinders in at least one mode of operation.
 14. Anengine method, comprising: opening exhaust valves of a first cylindergroup for a first duration; opening exhaust valves of a second cylindergroup for no more than a second, shorter duration between successivevalve closings; opening intake valves of the first cylinder group andopening intake valves of the second cylinder group for an equalduration; selectively routing exhaust from the first cylinder group toan exhaust passage via a first valve and to an intake passage via asecond valve; routing exhaust from the second cylinder group only to theexhaust passage; and maintaining an equal air-fuel ratio among allcylinders of both the first and second cylinder groups.
 15. The methodof claim 14, wherein selectively routing exhaust from the first cylindergroup to the intake passage and routing exhaust from the second cylindergroup only to the exhaust passage increases exhaust backpressure of thefirst cylinder group relative to the second cylinder group, and whereinthe exhaust valves of the first cylinder group are opened for the firstduration and the exhaust valves of the second cylinder group are openedfor no more than the second duration in order to match airflow ratesthrough the first cylinder group and the second cylinder group.
 16. Themethod of claim 14, wherein opening exhaust valves of the secondcylinder group for the second, shorter duration further comprisesclosing the exhaust valves of the second cylinder group earlier than theexhaust valves of the first cylinder group.
 17. A system, comprising:means for routing exhaust gas from a first cylinder group of an engineto an exhaust gas recirculation passage coupled to both an intakepassage and an exhaust passage of the engine, the first cylinder grouphaving a first amount of positive intake and exhaust valve overlapresponsive to an exhaust backpressure of the first cylinder groupexceeding an exhaust backpressure of a second cylinder group of theengine, the first cylinder group having a second, smaller amount ofpositive intake and exhaust valve overlap responsive to the exhaustbackpressure of the first cylinder group being equal to the exhaustbackpressure of the second cylinder group; and means for routing exhaustgas from the second cylinder group of the engine only to the exhaustpassage of the engine, the second cylinder group having the second,smaller amount of positive intake and exhaust valve overlap, the exhaustgas recirculation passage configured to provide exhaust gas to both thefirst cylinder group and the second cylinder group.
 18. An enginemethod, comprising: during a first condition that includes an exhaustbackpressure of a first cylinder group of an engine exceeding an exhaustbackpressure of a second cylinder group of the engine, routing exhaustgas from the first cylinder group to an exhaust gas recirculationpassage coupled to both an intake passage and an exhaust passage of theengine, the first cylinder group having a first compression ratio duringthe first condition; during both the first condition and a secondcondition that includes the exhaust backpressure of the first cylindergroup being equal to the exhaust backpressure of the second cylindergroup, routing exhaust gas from the second cylinder group of the engineonly to the exhaust passage of the engine, the second cylinder grouphaving a second compression ratio, different than the first compressionratio; and during the second condition, routing exhaust gas from thefirst cylinder group of the engine only to the exhaust passage of theengine, the first cylinder group having the second compression ratioduring the second condition.